For security reasons, Google
apparently has decided to hide the
USB debugging activation on Nexus.

How to Enable USB debugging on Nexus (4,7,10) :

- Open up your device’s “Settings”. This can be done by pressing the Menu button while on your home screen
- Tapping “System settings” Now scroll to the bottom and tap “About phone”
or “About tablet”.
- At the “About” screen, scroll to the bottom and tap on “Build number” seven times. Make sure you tap seven times. If you see a “You are now a developer”
- message pop up, then you know you have done it correctly. Done. By tapping on “Build number” seven times, you have unlocked USB debugging mode on
Android 4.2 and higher.
- You can now enable/disable it whenever you desire by going to
“Settings” -> “Developer Options” ->
“Debugging” ->” USB debugging”.

By following the steps above. You can Enable USB Debugging On Nexus. (4,7,10).